Semir is the indisputably recognised star of the series, and is played by Erdogan Atalay.
His first appearance was in the first season's third episode alongside Frank Stolte, after the death of Ingo Fischer. Semir remains in the cast for all seasons up to those currently in production in Germany, while at his side alternating several colleagues, all of that soon become his friends. One short-lived exception is represented by Chris Ritter: Semir, in fact, initially sees him as the person who replaced the one he cared for more, Tom Kranich.
Semir is a character very much appreciated by the others in the series: he is always positive and never loses his cool. Often, he has to calm his rowdy colleagues, such as André Fux. Only in a few cases Semir can not keep calm: in the episode of Tom's death ("Life or Death"), he planned to threaten some people who have to deal with the death of his colleague. In "On Your Own," however, he must ensure justice for the murderer of Chris (a mafia leader known as Sander Kalvus); he tackles this case with much more determination and fury than he did in "Life or Death", to the point where he nearly kills Kalvus. In "Deadly Decision", he is forced to kill Ben if he is to see his family again, which terrifies Semir, to the point where he nearly committed suicide for that. He then shoots Ben at the top of the building, revealing his "death". However, Ben comes back to save the Gerkhans, and Semir reveals that the shot, where he used a blank-firing replica of his service pistol, was only a plan to deceive the gangsters and save his family as he severely injures Steffen Raiser as a reminder to never come after him again. He then exits prison, joyful to see his partner and his family are okay—especially that of his wife, Andrea.
In all other cases, however, he thinks before taking any action. Also, he does not hide his Turkish origin, which is shown in several episodes throughout the series.
Semir has also been recognized for driving his generic undercover patrol car: the BMW 3 series, which had alternately changed over the years, from the car itself (323i E36, 325ts E36, 328i E36, 330i E46, 330i E90, 320si E90 M-Sport, 320d F30, 328i F30 M-Sport) to the license plate number (HVL-S 199, UB-E 438, B-PV 185, NE-DR 8231, NE:DR 8231, D:BM 3308). If the BMW is wrecked during the investigation, he often chooses to either take another patrol car, steal a civilian vehicle, ride his partner's car, or just drive his partner's vehicle. Semir had also owned and used numerous exotic cars off-duty, but he had terrible events happen to them (BMW M735i wrecked, Ferrari F355 Spyder stolen).